An Mg alloy is light weight, gives strength at room temperature and high temperature, and is improved in corrosion resistance as well, so is being increasingly used for various applications. However, to improve the toughness as a structure and the plastic workability, the ductility has to be improved.
For example, Japanese Patent Publication (A) No. 2002-256370 proposes Mg100-a-bLnaMb, where Ln is at least one of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, and a misch metal, M is at least one of Al and Zn, 0.5≦a≦5, 0.2≦b≦4, and 1.5≦a+b≦7, where the crystal grain size is less than 2000 nm (=2 μm) so as to obtain high strength and high ductility. However, with a Zn content larger than 1 at %, the solid solubility limit in the Mg is exceeded, so Mg—Zn-based intermetallic compounds are produced and a high ductility is liable not to be realizable.
Further, Japanese Patent Publication (A) No. 5-306929 proposes Mgba1XaLnb, where X is at least one of Zn, Ni, and Cu, Ln is at least one of Y, La, Ce, and a misch metal, 1≦a≦10, and 1≦b≦20, where the average size of the crystal grains is 5 μm or less and the average grain size of the intermetallic compounds is 5 μm or less to provide strength, toughness, and secondary workability.
Japanese Patent Publication (A) No. 7-3375 proposes MgaZnbXc, where X is at least one element of Y, Ce, La, Nd, Pr, Sm, and a misch metal, 87 at %≦a≦98 at %, b and c are in the ranges shown in FIG. 1, 0≦Y≦4.5 at %, 0≦Ce, La, Nd, Pr, Sm, misch metal≦3 at %, where the microstructure is composed of a matrix phase of fine crystals in which Mg—Zn-based and Mg—X-based intermetallic compounds are dispersed so as to obtain high strength and high toughness.
International Patent Publication WO2004/085689 proposes including Zn in an amount of a at %, including at least one rare earth element selected from the group of La, Ce, and misch metals in a total of b at %, having a balance of Mg, with a and b satisfying the following expressions (1) to (3): (1) 0.2≦a≦3.0, (2) 0.3≦b≦1.8, and (3) −0.2a+0.55≦b≦−0.2a+1.95 so as to obtain a high strength and high toughness.
Japanese Patent Publication (A) No. 2005-113235 proposes Mg100-a-bZnaYb, where a/12≦b≦a/3 and 1.5≦a≦10, where the microstructure is an aged precipitated phase of Mg3Zn6Y1 quasi-crystals and their similar crystals dispersed in the state of microparticles so as to improve the high temperature strength.
Japanese Patent Publication (A) No. 2006-2184 proposes an Mg-based alloy containing 1 to 8 wt % of rare earth elements and 1 to 6 wt % of Ca and having a microstructure in which the maximum crystal grain size of Mg is 30 μm or less, the maximum grain size of intermetallic compounds is 20 μm or less, and the Mg is dispersed in the crystal grains and at the crystal grain boundaries so as to improve the strength and ductility at room temperature and the high temperature strength and fatigue strength near 200° C.
However, in each of the above, the difference in strength between the tensile deformation and the compressive deformation and ductility was not considered at all.